1. Field of the Invention
The present invention relates to an apparatus and method for exfoliation of graphene, and more particularly to an apparatus and method for exfoliation of graphene, in which graphene can be prepared by expanding graphite without a chemical treatment process using a strong acid like sulfuric acid and a thermal treatment process at a high temperature.
2. Description of Related Art
In general, graphite is a stack of two-dimensional graphene sheets having a layer structure in which carbon atoms are linked to one another in a hexagonal shape. Graphene is a representative flat single layer sheet in which three carbon atoms linked by an sp2 hybrid orbital bond are packed in a honeycomb crystal lattice.
In graphite, the carbon atoms in graphene are strongly linked to each other by a covalent bond, but graphenes are linked by a van-der-waals bond which is much weaker than the covalent bond.
Graphene is a single layer of graphite that is a (0001) plane of graphite. In graphite, since the bond between graphenes is weak, as described above, graphene has a very thin two-dimensional structure having a thickness of about 4 Å.
It has been found recently that graphene has very useful properties which are different from those of existing materials.
The most noteworthy property is that when electrons move in graphene, the electrons move as if the mass of the electrons is zero. This means that the electrons move at a speed at which light travels in vacuum, i.e., the speed of light. Further, another property is that graphene exhibits an abnormal half-integer quantum hall effect with respect to electrons and holes.
Moreover, the electron mobility of graphene known until the present is as high as about 20,000 to 50,000 cm2/Vs. Above all things, in case of carbon nanotubes which are similar to graphene, since production yield is very low if they undergo refinement after synthesis, the price of final product is too high even though the synthesis is performed by using a cheap material, but in case of graphite, it has an advantage in that the price thereof is very low. And in case of single-wall carbon nanotubes, the metallic and semiconductor properties vary according to chirality and diameter thereof, and also even if the semiconductor property is same, all of band-gaps thereof are different from one another. Therefore, each of the single-wall carbon nanotubes should undergo separation, which is difficult, in order to use a particular semiconductor property and a particular metallic property thereof.
On the other hand, in case of graphene, since the electrical properties thereof vary according to the crystalline directivity of graphene having a given thickness, a user can provide an electrical property in a direction which the user elects, and thus can easily design a device. Such electrical properties of graphene can be effectively used in carbon-based electric or electromagnetic devices and the like.
Due to such excellent properties of graphene, graphene receives attention as a substitute for next generation silicon and ITO (Indium Tin Oxide) transparent electrode and the like.
Many methods of manufacturing graphene have been continuously reported since 2004. Among them, there has been a method in which strong sulfuric acid or dangerous CIF3 is filled between layers of graphite and then treated at a high temperature so as to expand the graphite.
In this method, after a space between graphenes is expanded by pressure generated when volatile molecules such as sulfuric acid filled between graphenes, a thermal treatment process at a high temperature (of about 1,000° C.) has to be performed to remove various hydrophilic functional groups existing on the surface of graphite.
However, in this method, since a strong acid like sulfuric acid should be used and the thermal treatment process at a high temperature should be also performed, there is a problem that the above-mentioned excellent properties of graphene are deteriorated when graphite is exfoliated into graphene.
Moreover, when disposing of used sulfuric acid, environmental problems may be occurred, and the manufacturing cost thereof is increased due to the complicated thermal treatment process.